Refactor uniqueness check in Bitrix24PartnerRepository test contract

[mesilov]

Q	A
Bug fix?	no
New feature?	no
Deprecations?	no
Issues	Fix #468
License	MIT

Refactors the Bitrix24PartnerRepositoryInterface contract so save() stays a persistence operation and no longer requires repository implementations to perform an application-level duplicate bitrix24PartnerNumber check.

Changes included:

• removed testSaveWithTwoBitrix24PartnerNumber() from the repository contract test
• removed the duplicate-number preflight from the in-memory repository test implementation
• removed the @throws InvalidArgumentException contract annotation from save()
• updated CHANGELOG.md

Test plan

• make oa-schema-build — passed
• make test-file path=tests/Unit/Application/Contracts/Bitrix24Partners/Repository — passed
• make lint-cs-fixer — passed
• make lint-rector — passed
• make lint-phpstan — passed
• make lint-deptrac — passed
• make test-unit — passed

Closes #468

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